Pulsing NDIR Miniature Vacuum Lamps to Extend Their Life

Most NDIR miniature and sub miniature vacuum lamps are rated for 5V, AC, continuous operation. Lamp life ratings are determined by measuring a very large batch of lamps and recording when 50% of the lamps fail to illuminate.


Often to extend the life of the lamp, a custom circuit will be designed to convert from continuous to pulsed operation. By pulsing the lamp (switching it off and on), the amount of time the lamp is on during sampling is reduced, thereby increasing the lamp life.


However simply pulsing the lamp without insuring proper design to protect from inrush current can actually cause premature failure.  When the filament is cold and has much lower resistance, thermal shock can cause a fracture in the filament. This happens when power is applied and there is a surge of current which causes rapid expansion of the tungsten, along with thermal shock. Over time, the filament becomes thinner and more brittle and eventually experiences premature failure due to in rush current damage.

NDIR lamp filament

When designing the lamp power circuit there are a few commonly used methods to prevent damage from inrush current.  The first is called  simmer voltage. When using simmer voltage, a small amount of power is supplied to the lamp in the off cycle; typically just enough to keep the filament warm and reduce the impact of inrush current.  

 

inrush current

The second is to apply power at increasing levels until the peak /optimal light output level is reached, never applying the full power all at once.


And the third is to add a circuit that prevents the lamp from drawing additional current, or restricts the amount of additional current that can be drawn at start up. 

 

Life hours is a critical factor when selecting an NDIR lamp.  Understanding all contributing factors that affect a lamp's life, coupled with proper system design can greatly impact the life of both the lamp and the system it is designed into.  For more helpful information on NDIR lamps, visit our Technical Notes page here.